Add convenience function to look up a single value in a `DataFrame`

It would work similarly to @rsubset but it would automatically call only and extract the desired value.

For example:

julia> df = DataFrame(x = 1:5, y = ["A","A","B","B","B"], z = (1:5)./10)
5×3 DataFrame
 Row │ x      y       z
     │ Int64  String  Float64
─────┼────────────────────────
   1 │     1  A           0.1
   2 │     2  A           0.2
   3 │     3  B           0.3
   4 │     4  B           0.4
   5 │     5  B           0.5

julia> zvalue = @rsubset(df, :x > 1, :y == "A").z |> only
0.2

julia> zvalue = @lookup(df, :z, :x > 1, :y == "A")
0.2

It could maybe also work without the second argument to return a DataFrameRow.

It was mentioned that this may need to be called rlookup to match rsubset, but I think that is not necessary if the single value method is implemented. (This discussion started in the DataFrames.jl repository: https://github.com/JuliaData/DataFrames.jl/issues/3051#issuecomment-2059978408)

Would this function make DataFrames lookups more accessible to newcomers, or is the existing @rsubset functionality good enough?

Review the existing @rsubset functionality and the linked DataFrames.jl discussion first. Resolve whether the feature should be @lookup or rlookup, define behavior with and without the column argument, and confirm completion with tests covering the example lookup and DataFrameRow result.